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Welding-stainlesssteels and their properties:SOLUTIONS with Effective, Powerful Advice
What is in here for me? Welding-stainless steel requires basic knowledge of stainless steel types, classes or groups. They are called austenitic, ferritic, martensitic, precipitation hardenable or duplex according to the prevailing microstructure or arrangement of their crystal structure. Welding-stainless steel should not be complicate, we agree. But before talking of the welding processes we should know some more on the metals called by this name.
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Although some may think of stainless steel as a well defined material, in effect there are so many different types and sub-species, that just keeping track of them could be a little confusing. Therefore trying to put some order in the field might be helpful. This is what we intend to do by providing a short description of the different types, that will help in understanding their main characteristics. What benefits can be found here? After a short overview of Welding-stainless steels, tips on how one welds each type will be presented. However as types and conditions of various stainless steels can be very different, so will be the problems and their solutions. Before dealing in detail with Welding-stainless, one should offer a loose description of stainless steels in general. They are a class of iron base materials which have a certain resistance to rusting and to corrosion in some environments, due to the presence, in their composition, of at least 12% of the element Chromium. The reason of this behavior is that chromium helps produce on the material's surface a tough and impervious layer of chromium oxide, which is the shield protecting the surface from rust and corrosion. By now it should be clear that, when dealing with Welding-stainless, the expression STAINLESS STEEL represents a huge class of different iron base materials, having in common only some chromium content. It is not a technical term, it cannot identify any specific metal, and therefore it cannot be used for any practical purpose i.e. for purchasing. Three main classes... The three more general classes of stainless steels (and a mixed one called Duplex) are indicated, as hinted above, by reference to their metallurgical micro-structure. The use of the names of the classes, called "Austenitic", "Ferritic", "Duplex" and "Martensitic", sheds at least some information on some of their characteristics and uses. Such identifiers refer to the appearance of their micro-structure as seen under the microscope, after suitable preparation, or by another very special technique called x-ray diffraction. The microstructure of course has influence on their properties and on their Welding-stainless characteristics. It should be clarified that the microstructures identified by those names may be present at the same time in various proportions in a certain steel, and that therefore they are used to indicate the prevailing structure.
It should be understood that it is mainly the chemical composition of the specific material that determines the prevailing microstructure obtained. What is Austenitic... When Welding-stainless is involved, Austenitic stainless steels are considered the most weldable of their category. Common chromium-nickel austenitic stainless steels are known as the "300 series". This refers to a standard classification originated by AISI - American Iron and Steel Institute and by SAE - Society of Automotive Engineers. An important sub-class is also known as "18/8" (meaning that the important alloying elements of these steels are about 18% Chromium and 8% Nickel). Another sub class known as the "200 series" is identified by its composition including manganese-chromium-nickel-nitrogen. Specialty alloys with somewhat different compositions may be also included in the austenitic class Main characteristics of Austenitic stainless steels are as follows:
In Welding-stainless, these two last characteristics variously affect the outcome, producing larger distortion than as found in other steels. Not all austenitic stainless steels of the 300 series are equally well weldable. The addition of sulfur or selenium used to improve machinability (as in Type 303) results in severe weld hot-cracking, which makes this particular material "non-weldable". Austenitic stainless steels are used in the most various applications for industrial plants and consumer appliances, and also for food processing and pharmaceutical equipment. The main problems of Welding-stainless steels are the prevention or mitigation of sensitization, of the heat affected zone (HAZ), and hot cracking that depends on the structure developing in and around the weld metal. Beware of... The corrosion resistant characteristics of stainless steels may be adversely affected by heating in the temperature interval from about 600 to 900 °C (from about 1100 to 1650 °F). This process called sensitization, occurring while Welding-stainless steel, promotes the precipitation (gathering) of chromium carbides at grain boundaries. Note: Grains are units consisting of identical crystals all in the same orientation. Boundaries separate grains from one another. Sensitization means that a certain portion of the metal becomes depleted of the element chromium, therefore losing its anticorrosive properties. Sensitized material is therefore susceptible or prone to corrosion. At the Welding-stainless location, temperature is higher and lasts only for a short time. The above dangerous range of temperatures occurs naturally in two strips of metal on both sides of the weld bead. This is the so called Heat Affected Zone (HAZ) where the harmful effects take place. In a sensitized joint, as explained above, the chromium, which is the main "stainless" ingredient, becomes sequestered or taken out of play and locally unavailable for the protective action. If not addressed correctly, Welding-stainless 18/8 steels may thus cause the loss of their protective property along sensitized paths. The welded material becomes prone to intergranular attack in a corrosive environment. The Big Three... Three strategies are usually employed to oppose this tendency. One is to use a very low carbon version (i.e. 304L where L stands for low-carbon) where not much carbon is available for making chromium carbides. Another is to use a different type of base metal including an amount of titanium (type 321) or columbium (also known as niobium) (type 347). These elements tend to form readily titanium carbides (or columbium carbides) (and by this action the carbon becomes unavailable for chromium) at sensitization temperatures, leaving the chromium free to perform its anticorrosive task. Note that the filler metal for this material, if required, should be always of type 347. Why? Because titanium (in type 321) being reactive, is not readily recovered during deposition, so that it would not be available when it is needed most. Columbium however is not reactive, it will stay put through melting, and, when the material is heated to the "sensitization" temperature, will do its job of producing columbium carbides in preference to chromium carbide, and so it will save the day. The third strategy for safely Welding-stainless is to perform a solution heat treatment at elevated temperature (about 1050 °C or 1900 °F), for repairing a condition of corrosion susceptibility. This puts again in solution (called solid solution) the chromium carbides originated during Welding-stainless sensitization of regular 18/8 stainless (like types 302 or 304). This process however must contend with problems of heavy oxide formation if not done in vacuum or protective atmosphere, and of distortions. Type 309 and 310, used for elevated temperature applications, and type 316 or better type 316L used for enhanced corrosion resistance, are generally not prone to sensitization and are used with filler wires of similar composition. Now Ferritic... Other stainless steels, called Ferritic, are ferromagnetic but cannot be hardened by heat treatment. A limited amount of ferritic structure, when present in an otherwise mainly austenitic structure, is considered beneficial in that it reduces the chances of hot cracking. Welding-stainless ferritic steels can readily be performed using arc processes, either with ferritic or with austenitic filler metal, except that a post weld heat treatment may be needed for improving properties. Car exhaust components may be made out of these materials. Then Duplex... Duplex means double and, when referred to Stainless Steel, it indicates a mix in about equal proportions, of the two kinds of metallographic structures just introduced, called by metallurgists ferrite and austenite. Compositions are identified with reference to the Unified Numbering System. These are Stainless Steels with quite a substantial proportion of Chromium, the main ingredient for imparting stainless properties, and additional balanced quantities of Nickel, Molybdenum, Copper and sometimes Nitrogen. The base metal is obviously Iron. Carbon is kept low as are also Sulfur and Phosphorus. The main properties are improved corrosion resistance and mechanical properties (up to twice as strong), when compared with regular austenitic stainless steels. Other important differences are thermal conductivity and thermal expansion, midway between those of carbon and austenitic stainless steels. Although weldability of Duplex steels is good, attention should be devoted to sensitivity to degradation of properties, if exposed at heat for excessive time. An important precaution for Welding-stainless of this type is to limit as much as possible holding time at intermediate temperature, between 300 and 980 °C (570 and 1800 °F). Filler metals are chosen either with matching compositions or overalloyed with slight excess of nickel to promote more austenitic structure. Thorough documentation is recommended before starting a project and full qualification following procedure development, to assure acceptable properties in the complex structures that benefit most of the use of Duplex Stainless Steels.
And Martensitic... Martensitic stainless steels are magnetic and fully hardenable by heat treatment. Welding-stainless of this type is not recommended, although feasible with special techniques. Welding may produce cracks, especially if carbon content is not sufficiently low. Preheat and postheat may be necessary. One more class... To complete the Welding-stainless overview, one should mention a fourth class of materials, not listed above, called precipitation hardenable (PH) stainless steels, which are quite readily weldable. However precise instructions should be followed concerning heat treatments in order to develop the required properties. Note: - Always check the last edition of AWS Documents by downloading the current AWS Publications Catalog from
The following AWS Documents provide guidance in activities relative to Welding Stainless Steels:
D1.6/D1.6M:2007, Structural Welding Code—Stainless Steel ANSI/AWS D10.4-86(R2000)
ANSI/AWS D18.1/D18.1M:2009 ANSI/AWS D18.2:2009
The Processes... FRICTION Welding-stainless steels presents almost no problems, except for the free cutting types that should not be welded at all. It is used for Welding-stainless steel not only to itself but also to quite different materials like copper or aluminum and other combinations. One should always be aware
Any of these conditions could compromise the final soundness of welded joints. RESISTANCE process for Welding-stainless is currently used. Due adaptations from schedules used for carbon steels derive from differences in high electrical resistance and low thermal conductivity, high coefficient of thermal expansion, higher melting temperature and higher strength at elevated temperature. Electrode force is more elevated, while time and current are less than for low carbon steels. Resistance Welding-stainless Austenitic steel of the 300 series is readily performed. Ferritic steels are also welded, but martensitic, hardenable, stainless steels are problematic as the welds result brittle, if not softened adequately by a post weld tempering treatment. Stainless steels must not only be cleaned from external surface dirt, oil, grease or paint before welding but also from the naturally forming chromium oxide layer which should be removed with a stainless steel wire brush. Oxyacetylene GAS WELDING could be used for Welding-stainless steel but the use of a proper flux is required. This makes the process much less attractive than Gas Tungsten Arc Welding (see in the following) unless there is no other choice. It is imperative to eliminate all traces of residual flux on the part after welding, to avoid corrosion attack: this introduces an additional operation which might increase the cost. ARC WELDING is commonly used for Welding-stainless steel with due attention being paid to the class and to the condition of the material and to the influence of the process on such consequences as sensitization or deformations. All the main arc welding processes can be used with suitable consumables. Shielded Metal Arc Welding (SMAW) with covered electrodes, Gas Tungsten Arc Welding (GTAW or Tig), Gas Metal Arc Weld (GMAW or Mig) with consumable continuous filler wire, Flux Cored Arc Welding (FCAW) and Submerged Arc Welding (SAW). To learn more, procure the following Book
Filler metals... On Stainless Steels Filler Metals, the following AWS Specifications can be found
ANSI/AWS A5.9/A5.9M:2012
ANSI/AWS A5.4/A5.4M:2006
ANSI/AWS A5.8M/A5.8:2011
ANSI/AWS A5.22/A5.22M:2012
ANSI/AWS A5.31M/A5.31:2012
An Article on the Selection of Stainless Steel Filler Metal was included in our Practical Welding Letter No. 02 of October 2003. An Article on Filler Metals for Duplex Stainless Steels was included in the January 2005, Issue # 017 of Practical Welding Letter. An Article on tips for Welding thin Stainless Steel Sheets was published (3) in the Issue 32 of Practical Welding Letter for April 2006. An Article on Corrosion in Weldments was published (7) in the Issue 32 of Practical Welding Letter for April 2006. To read it click on PWL#032. An Article on Welding Effects of S Content on Stainless Steels was published (11) in issue 45 of Practical Welding Letter for May 2007. An Article on Welding Defects in Stainless Steels was published (11) in Issue 48 of Practical Welding Letter for August 2007. An Article on Ferrite in Austenitic Stainless Steels was published (7) in Issue No. 53 of Practical Welding Letter for January 2008. A Contribution titled Beware of 317LMN Castings! was published (11) in Issue No. 53 of Practical Welding Letter for January 2008. An Article on Pitting Resistance Equivalent Number was published (7) in Issue 54 of Practical Welding Letter for February 2008. An Article on Stress Corrosion Cracking (SCC) was published (7) in Issue 61 of Practical Welding Letter for September 2008. An Article on Welding-Stainless Steel Castings was published (2) in Issue 64 of Practical Welding Letter for December 2008. An Article on Spot Welding Duplex Stainless was published (11) in Issue 81 of Practical Welding Letter for May 2010. An Article on Welding Stainless to Nickel alloy 600 was published (3) in Issue 81 of Practical Welding Letter for May 2010. An Article on New Chromium Free Filler Metal for Joining Austenitic Stainless Steel was published (4) in Issue 93 of Practical Welding Letter for May 2011. An Article on Filler Metals for Welding Dissimilar Materials used at High Temperatures was published (4) in Issue 95 of Practical Welding Letter for July 2011. An Article on Filler Metal Pyromet® X-23 was published (4) in Issue 98 of Practical Welding Letter for October 2011. An Article on Large Power Generation Components was published (11) in Issue 102 of Practical Welding Letter for February 2012. An Article on Constitution Diagrams was published (11) in issue 108 of Practical Welding Letter for August 2012. An Article on Filler Metals for SAW cladding with 309LMo was published (4) in Issue 110 of Practical Welding Letter for October 2012. An Article on Welding Wrought Precipitation Hardening Stainless Steels - I - Martensitic PH Stainless Steels An Article on Welding Wrought Precipitation Hardening Stainless Steels - II - Semiaustenitic PH Stainless Steels An Article on Welding Wrought Precipitation Hardening Stainless Steels - An Article on Superferritic Stainless Steels was published (2) in issue 115 of Practical Welding Letter for March 2013. An Article on Welding Cast Stainless Steels (2) and another on An Article on Duplex Stainless Steels - An Article on More on Duplex Stainless Steels was published (2) in Issue 122 of Practical Welding Letter for October 2013. An Article on Helpful Information on Welding Stainless Steels Standards was published (7) in Issue 130 of Practical Welding Letter for June 2014. An Article on Activated TIG Welding was published (11) in Issue 135 of Practical Welding Letter for November 2014. An Article on Precautions when welding Superferritic was published (2) in Issue 140 of Practical Welding Letter for April 2015. An Article on Filler Metal for GTA Welding 304L to 904L was published (4) in Issue 146 of Practical Welding Letter for October 2015. An Article on Filler Metal 312 failing transverse side bend tests was published (4) in Issue 150 of Practical Welding Letter for February 2016. An Article on A new method of brazing stainless steel parts was published (7) in Issue 151 of Practical Welding Letter for March 2016. An Article on Duplex Stainless Filler Metals and Procedure Qualification Problems was published (4) in Issue 152 of Practical Welding Letter for April 2016. An Article introducing Bulletin_119 on Stainless Steels was published (8) in Issue 154 of Practical Welding Letter for June 2016. An Article on Welding Austenitic SMO 254 Stainless Steel was published (4) in Issue 155 of Practical Welding Letter for July 2016. An Article on Filler Metal for Stainless Steels was published (4) in Issue 156 of Practical Welding Letter for August 2016. An Article on Comparing measured and calculated Ferrite content was published (4) in Issue 158 of Practical Welding Letter for October 2016. An Article on FILLER METAL for welding super duplex stainless was published (4) in Issue 160 of Practical Welding Letter for December 2016. An Article on Using flame to redress distortions in stainless was published (3) in Issue 164 of Practical Welding Letter for April 2017. An Article on Improvements in Vacuum Furnace Design was published (7) in Issue 167 of Practical Welding Letter for July 2017.
To receive regularly by e-mail at no cost the periodic publication above, please Subscribe. All types of arc processes can be employed for Welding-stainless steels with due attention to joint shape, dimensions and preparation. In particular Shielded Metal Arc Welding (SMAW) is widely used because of its flexibility. It should be noted that electrodes come in two types concerning the cover composition, which may influence the choice of the current employed:
ELECTRON BEAM Welding of stainless steels is readily performed with good results even in very deep welds. As usual the remarkably high depth to width ratio permits to join configurations not possible with other means. The heat input being low and the heat affected zone of limited extent, there is often no remarkable damage to the mechanical properties so that further heat treatment is not required. Also Laser Beam Welding is performed for Welding-stainless steels, with the usual precautions needed to insulate the weld from air and to limit the damage to properties obtained by heat treatment. NEW RESOURCES
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